Terry Pratchett, the bestselling author of the Discworld fantasy books, is suffering from a rare form of early onset Alzheimer's.

The author has published a statement on a website calling the diagnosis "an embuggerance". Pratchett, who is 59, says that he is taking the news "fairly philosophically" and "possibly with mild optimism". He adds that the statement, posted yesterday on the website of his illustrator Paul Kidby, "should be interpreted as 'I am not dead'" and says that he expects to meet most current and, as far as possible, future commitments.

Earlier this year Pratchett underwent medical investigations after he started having problems with hand-eye coordination and dexterity. An MRI scan showed some areas of dead tissue and the suggestion was that he had suffered a "mini-stroke" some time in the past few years, and that he was now living with its legacy. In his statement, Pratchett says that the early onset Alzheimer's "lay behind this year's phantom 'stroke'".

Pratchett is one of the country's most popular authors. He has sold more than 55m books and is best known for his Discworld series of humorous and satirical fantasy novels. The 36th of these, Making Money, was published earlier this year. He has also written children's books, including The Amazing Maurice and His Educated Rodents which won the prestigious Carnegie medal for children's fiction. He was awarded an OBE in 1998 for services to British literature.